Love and correspondence before marriage
Correspondence between the sexes is not permissible, because that provokes temptation and usually results in evil. If a man corresponds with a non-mahram woman in letters that are not seen by anyone else, that leads to many evils. Islam forbids a woman to be alone with a man who is not her mahram because of the fitnah (temptation) and bad things that result from that, such as attachment and the desire to look and touch, etc. All of this results from the man talking to the woman in these private letters or conversations, especially if they are young and at an age when desire is strong.
Shaykh Ibn Jibreen (may Allaah preserve him) was asked: What is the ruling on correspondence between young men and young women, if this correspondence is free from immorality, love and desire?
He replied: It is not permissible for any person to correspond with a woman who is not his mahram, because of the temptation involved in that. The person may think that there is no temptation, but the shaytaan will keep trying until he tempts him through her, and tempts her through him. The Prophet(peace and blessings of Allaah be upon him) told the one who hears of the Dajjaal to keep away from him, and he said that a man may come to him as a believer but the Dajjaal will keep trying until he tempts him and confuses him.
There is a great deal of temptation and danger in correspondence between young men and women which means that it has to be avoided, even though the questioner says that there was no love or desire involved. End quote from Fataawa al-Mar’ah, compiled by Muhammad al-Musnid, p. 96.
The woman is forbidden to speak softly to one who is not permissible for her, as Allaah says (interpretation of the meaning):
“be not soft in speech, lest he in whose heart is a disease (of hypocrisy, or evil desire for adultery) should be moved with desire, but speak in an honourable manner”
And it is haraam for a man to enjoy listening to the speech or voice of a non-mahram woman.
Making your sister the go-between in passing on the letters is another evil, because that is encouraging her to engage in this kind of activity.
Based on this, what you have to do is repent to Allaah, and do a lot of righteous deeds, but you do not have to do anything else.
And Allaah knows best.